It means that there is always a higher point, also if we can't imagine it.
There is also a higher point than Pluto('s robot).
But the highest point can only be on top, that's why .top = top.
The matter of that fact is, that .top can be expressed as the highest point while no other point can be expressed as that. The top have to do with everything below the top (like the last stone of a pyramid - it is the 'result' of all other stones).
The TLD .top is the 'result' of all other TLD's - everything 'must have an end somewhere', there are enough TLD's in the www now and .top is the 'final TLD'.